1. Field of the Invention
The present invention relates, in general, to an liquid radioisotope, e.g. F-18, production target having an internal support, which produces a radioisotope F-18 and, more particularly, to an F-18 production target having an internal support, in which the deformation of thin sheets, which occurs toward the center of an H218O holder, is reduced, thereby increasing the durability and the lifespan thereof.
2. Description of the Related Art
Generally, a target system for producing radioisotopes refers to a system that changes the state of matter of stable isotopes so as to receive high-energy protons accelerated at a cyclotron, to cause a nuclear reaction with stable isotopes, and to convert the stable isotopes into radioisotopes.
The target system for producing the radioisotopes is divided into three target systems: solid, liquid and gaseous, according to the state of matter of stable isotopes. Among them, the liquid and gas target systems are designed in a hermetic type in order to prevent the produced radioisotopes from leaking outside.
In particular, the liquid target system is widely used because it produces a great deal of isotopes via a nuclear reaction, and maintains a liquid phase, which is very advantageous in synthesizing various isotopic compounds. The radioisotopes produced using this target system are applied to the diagnosis of tumors or cancer.
Various methods of diagnosing tumors or cancer have been developed and used, such as X-ray computed tomography (CT), magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), positron emission tomography (PET), and so on.
Above all, the PET technology is technology for injecting a radioisotope or a labeled compound, emitting a positron, into a living body, and then imaging the distribution of the injected material in the body. The X-ray CT or the MRI images a structure in the human body to anatomically diagnose lesions, whereas the PET diagnoses abnormalities in the body using biochemical changes occurring prior to anatomical changes in the event of the onset of a disease.
Among radioactive medicaments used to obtain an image of the PET, one called 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) (hereinafter, referred to as “FDG”) synthesizing fluorine (F) into glucose is widely used. A radioisotope, F-18, used for synthesizing FDG is produced by irradiating high-energy protons generated by the cyclotron onto H218O to thereby cause a nuclear reaction 18O(p,n)18F.
In detail, as in FIG. 1, F-18 isotopes are produced by causing the nuclear reaction 18O(p,n)18F adopting O-18, an isotope of O-16, as a target material using the protons accelerated by the cyclotron. In other words, O-18, receiving the protons, emits neutrons, and is then converted into F-18.
F-18 is estimated to be the most ideal nuclide for use in the nuclear medical field because it decays by positron (β+) emission and has a half life of 110 minutes. Further, F-18 has a characteristic such that it is capable of obtaining a high resolution image because it has maximum positron energy of 511 keV and an average range of 2.4 mm in water.
Also, F-18 has a relatively long half life compared to other PET nuclides, so that it can have a long enough lifespan to synthesize the medicaments containing F-18, and so that it is appropriate to measure changes in distribution and concentration of these medicaments in a living body over time.
F-18 has a size similar to that of hydrogen, so that it does not greatly change the geometrical structure of a molecule (of another element). However, F-18 has much stronger electronegativity than hydrogen, and greatly increases lipophilicity, that is, affinity to fat, so that a great physical, biochemical change occurs in the molecule.
Part of the energy of each proton for this nuclear reaction 18O(p,n)18F is absorbed to a thin sheet, and is responsible for an increase in temperature of the thin sheet. The heated thin sheet is cooled using coolant or gas such as helium (He).
The target system for these radioisotopes is disclosed in Korean Patent Nos. 10-0293690 and 10-0278585. This conventional target system is illustrated in FIGS. 2 and 3.
As illustrated in FIGS. 2 and 3, the conventional target system 1 comprises a frame 10, which is formed with steps 14 in the front and rear inner circumferences thereof, flat faces 15 extending from the steps 14 in a radial inward direction, a predetermined space into which H218 O, containing a stable isotope O-18, is introduced and held in a central portion 11 thereof, and through-holes 12 and 13 communicating with the central portion 11 such that H218O can flow in and out in a diagonal direction, and thin sheets 20, which are welded to the flat faces 15 of the frame 10 on opposite sides of the central portion 11 of the frame 10 such that H218 O does not leak out of the front and rear of the central portion 11.
Further, in order to prevent H218 O, held in the central portion 11, from leaking outside, ring-shaped seal members made of polyethylene (PE) may be selectively interposed between the thin sheets 20 and the frame 10.
In other words, the seal members 40 are compressed between the thin sheets 20 and the frame 10, so that they can prevent H218O from leaking to the outside.
The material held in the central portion 11 is H218O, the mass of which is basically equal to that of water. The proton accelerated by the cyclotron is characterized in that energy is abruptly reduced depending on the density of material. Thus, the target system 1 for producing isotopes is designed using essential components so as to be able to maintain the energy of the proton unchanged.
For this reason, the metal thin sheet 20 is used at the front of the target system through which the proton accelerated by the cyclotron passes. The target system 1, developed in Korea, is adapted so that it employs these metal thin sheets 20 on opposite sides thereof so as to conduct smooth cooling.
This convention target system 1 is filled with H218O at the central portion 11 of the frame 10. In the case in which the protons are irradiated onto H218O, the central portion 11 enters a high-pressure state due to heat generated by the nuclear reaction. At this time, the generated pressure is higher than the pressure of the coolant circulating around the thin sheets 20, and thus the thin sheets 20 are deformed in outward directions, as in FIG. 4A This deformation causes the level of the liquid in the central portion 11 to be lowered, so that the loss of the protons occurs.
In order to solve this problem, most research institutes or commercial companies make undertake research, so that separate grid structures are installed outside the respective thin sheets 20 so as to minimize the deformation of the thin sheets 20.
These grid structures are adapted to be installed outside the respective thin sheets 20 so as to prevent the thin sheets 20 from being deformed in outward directions. Each grid structure has a disc shape, and is provided with a plurality of through-holes in the central portion thereof such that the protons pass through the through-holes to be irradiated onto the central portion 11 of the frame 10.
However, although the aforementioned grid structures are installed, the thin sheets 20 are deformed inward toward the central portion 11 as in FIG. 4B due to the pressure of the external coolant in the process of recollecting the liquid after the protons are irradiated or in the state in which the central portion 11 is emptied.
Since the thin sheets 20 cause permanent deformation by means of external pressure or weak force, the magnitude of the permanent deformation is increased in proportion to the number of times that the target system 1 is used, so that the thin sheets 20 shrink.
Specifically, a small amount of H218O is loaded in the process of loading? O-18, and then the protons are irradiated. Thereby, the thin sheets 20 are deformed outwards due to heat and pressure, so that the level of H218O is lowered.
This result leads to problems of the loss of the protons occurring at the target system 1 before the grid structures are installed and of cooling insufficiency caused by a decrease in the cooling area. As this deformation is repeated, the magnitude of the deformation is increased. Ultimately, this acts as a main factor that reduces the life span of the target system 1 and the production yield of the isotopes.